1. Field of the Invention
This invention relates to a fuel feeding system for an internal combustion engine, and particularly to a fuel feeding system having a carburetor for providing a fuel-air mixture, a fuel injection equipment, etc., and more particularly to a fuel feeding system having a carburetor for supplying fuel-air mixture (emulsion) to an engine of an automobile, a motor-bicycle, a bicycle having a motor, a pocket motorcycle, an outboard motor, a hang glider, a chain saw, a lawn mower, a road-cutter, etc., a fuel injection equipment, a fuel injection nozzle, etc.
2. Description of Related Art
A carburetor has been known as one of fuel feeding systems for mixing air and fuel in a suitable mixing ratio and then supplying air-fuel mixture to an engine of an automobile, etc. for combustion. A conventional carburetor is provided with a throttle valve disposed in an air-suction passageway so as to be movable in such a direction that air flow in the air suction passageway is suitably intercepted to form a variable venturi portion in the air-suction passageway, a fuel feeding passageway which serves to control a fuel flow (supply) amount to the venturi portion and is intercommunicated to the air suction passageway so as to be intersected to the air-suction passageway, and a tapered jet needle whose diameter is gradually reduced toward its tip portion, the rear end portion of the tapered jet needle being secured to the throttle valve while the front (tip) end portion thereof is inserted into the fuel feeding passageway. In the carburetor thus constructed, the clearance (gap) between the jet needle and the fuel feeding passageway is varied by suitably moving the throttle valve in the intersecting direction to the air suction passageway, and the fuel amount proportional to an air suction amount flowing in the venturi portion is fed to the venturi portion with controlling an air fuel ratio.
In general, the tip portion of the jet needle has a needle-shaped portion which is tapered with a constant gradient, or a conical portion which is tapered with a gradient being varied at the tip portion of the conical portion. The conical tapered jet needle generally has a vertical angle of about 60 degrees.
Further, the surfaces (walls) of the fuel feeding passageway and the jet needle along which the fuel flows are smoothly formed (smoothened) to reduce flow resistance between the fuel and the surfaces (walls). That is, each of the fuel feeding passageway and the jet needle has a smoothened or flat surface.
In this type of carburetor, when the jet needle is moved rearwardly (in such a direction that the intercommunication between the air suction passageway and the fuel feeding passageway is opened) to broaden the clearance between the jet needle and the fuel feeding passageway, the jet needle is liable to be fluctuated due to vibration of an engine, or to be downwardly pushed by air pressure in the air suction passageway, so that a fuel feeding state in the venturi portion is instabilized and thus the stability of the air fuel ratio is lost. Therefore, in the conventional carburetor having the fuel feeding system as described above, a knocking phenomenon due to reduction in combustion efficiency and a time lag to accel response (so-called discontinuous combustion) frequently occur, so that an engine efficiency is greatly reduced. The reduction of the engine efficiency causes a moderate or dull power-up of horsepower at a lower speed region (thus causes reduction in starting power), and the discontinuous combustion causes a rapid speed change (thus a violent fall of a motorbicycle, etc.).
In order to overcome the above disadvantages, the Japanese Laid-open Patent Application No. 59-90751 has proposed a fuel feeding system in which the outer diameter of a jet needle is set to be substantially equal to the inner diameter of a needle jet constituting a fuel feeding passageway to prevent the jet needle from being fluctuated over a movable region of the jet needle, and a chamfered portion is formed at the side surface of the jet needle such that clearance between the needle jet and the inner surface of the needle jet is gradually increased toward the tip portion of the jet needle.
Conventional techniques directing an improvement in performance of the above type of fuel feeding system, which representatively contains the Japanese Laid-open Patent Application No. 59-90751 as described above, have been researched and developed to mainly prevent the fluctuation of the jet needle. In addition, in these conventional techniques, the tip portion of the jet needle has been commonly formed in a conical shape having an acute vertical angle in consideration of the basic concept of hydrodynamics that smooth flow of fuel can be obtained by reducing flow resistance of the fuel.
However, according to the consideration of the inventor of this application, the low combustion efficiency of the conventional fuel feeding system can be estimated not to be caused by the instability of the air fuel ratio due to the fluctuation of the jet needle, but to be caused by the following two points.
Firstly, the low combustion efficiency would be caused by the smoothened (flat) wall surface of a fluid passageway such as a fuel feeding passageway, an air suction passageway, etc., along which fuel, air or air-fuel mixture flows in contact with the wall surface thereof although the smoothened surface itself is considered as a most preferable surface on the basis of the hydrodynamics. That is, since the wall surface of the fuel feeding passage or the surface of the jet needle (hereinafter referred to as "wall surface") is smoothly (flatly) formed in a conventional fuel feeding system, a boundary layer is formed between the surface wall of each of the fuel feeding passageway and the jet needle and the fuel due to friction therebetween. The flow of the fluid such as fuel, air or air-fuel mixture is decelerated by the boundary layer, so that the fuel feeding is restricted or disturbed. This restriction or disturbance of the fluid flow by the boundary layer mainly causes the instability of the air fuel ratio. Therefore, the conventional fuel feeding system can not provide an ideal air combustion ratio. In addition, difficulty in increase of air suction amount for power-up would be also caused by the smoothly-formed (smoothened) surface wall of the air suction passageway. When the clearance between the jet needle and the fuel feeding passageway is small, the clearance would be mostly occupied by the boundary layer, and thus the flow resistance of the fuel would be remarkably great.
Therefore, if the area of the boundary layer is reduced in the fluid passageway, the flow condition of fuel, etc. could be approached to an ideal condition in which no friction occurs between the fluid (fuel, air, air-fuel mixture) and the wall surface of the fluid passageway, and thus the flow resistance could be reduced to increase the fuel feeding amount, so that an ideal (optimum) air fuel ration can be obtained to improve the combustion efficiency.
Further, conventionally, only the clearance between the jet needle and the fuel feeding passageway has been considered, but no consideration or attention has been paid to the flow resistance caused by the boundary layer, and thus it has been conventionally difficult to control the fuel flow amount in proportion to the clearance. Therefore, the design and setting of peripheral equipments of the jet needle have not been simply performed, and skilled sense and experience have been required for the design and the setting.
Secondly, the low combustion efficiency would be caused by stable fluidity of fuel which is controlled by the shape of the tip portion of the jet needle. That is, the fuel is allowed to smoothly flow through the clearance between the jet needle and the needle jet by an acute shape of the tip portion of the jet needle and thus the fluidity of the fuel itself is stabilized irrespective of the instability of the fuel feeding to the venturi portion. This stability of the fluidity of the fuel causes insufficient fine-granulation of the fuel in the venturi portion where the air-fuel mixture is generated and/or insufficient turbulence of the air-fuel mixture, so that a flaming speed in a combustion chamber can not be improved. Accordingly, if the stable fluidity of the fuel in the clearance between the jet needle and the needle jet is intentionally disturbed to form turbulent flow of the fuel in the clearance, the turbulent flow of the fuel would cause the turbulence of the air-fuel mixture and thus improve the combustion efficiency.